Deal with C++ incompatibility of sys_nerr declaration by taking it out

of c.h altogether, and putting it into the only places that use it
(elog.c and exc.c), instead.  Modify these routines to check for a
NULL or empty-string return from strerror, too, since some platforms
define strerror to return empty string for unknown errors (what a useless
definition that is ...).  Clean up some cruft in ExcPrint while at it.